This invention is directed to a beam deflection system for bending a charged particle beam and focusing it onto a target, and in particular it is directed to a doubly achromatic, double focusing magnet system.
In present therapy electron accelerators, it is usually necessary to have a bending magnet system which will bend an accelerator beam approximately 90.degree. onto a target. The geometry must be acceptably compact for a range of electron energies between 5 and 25 MeV. This geometry usually requires that the beam be bent back across itself resulting in a beam being deflected at an angle from 225.degree. to 280.degree..
Because of the broad energy spread of the electrons in the beam and the restrictions required on beam divergence angle on a target, a doubly achromatic system is necessary. In the Review of Scientific Instruments, Vol. 34, page 385, 1963, H. A. Enge describes a single magnet system which is doubly achromatic for bending a beam 270.degree.. However, this system would be difficult to manufacture and requires very accurate field mapping and shimming.
Standard doubly achromatic, double focusing systems are based on having a mirror plane of symmetry halfway through the magnet system. Examples of symmetric three-magnet systems are described in U.S. Pat. No. 3,691,374 which issued to Leboutet on Sept. 12, 1972; and U.S. Pat. No. 3,867,635 which issued to Brown et al on Feb. 18, 1975. An example of a four-magnet 180.degree. system is described in U.S. Pat. No. 3,967,225 which issued to E. A. Heighway on June 29, 1976. These systems have been found to have relatively large orbit dimensions, i.e. the perpendicular distance or height of the magnet system above the projected input axis.